Hydroconversion process of heavy oil and residuum is one of the main processes for converting a heavy hydrocarbonaceous feedstock to lower boiling products. Generally heterogeneous catalyst, such as alumina or silica-alumina supported sulfide of cobalt, molybdenum or nickel, is used in the process. The constituents having higher molecular weight in heavy oil and residuum deposit on the surface of the catalyst, block the pores of the catalyst, and then result in rapid decline of the hydrogenation activity. Eventually coke and metal impurities removed from heavy oil and residuum deposit on the surface of the catalyst and result in deactivation of the catalyst. Moreover, the rapid increase in pressure drop of the bed layer makes it difficult to maintain normal operation, which becomes more serious when the feedstock contains higher metal and carbon residue, thereby, the catalyst displays short service life and bad operation stability, therefore shut-down is more frequent.
In order to solve these problems, many dispersing-type catalyst have been proposed. Chinese Patent Application CN 1035836A discloses a dispersing-type catalyst and its preparation method, wherein iron compound (especially ferrous sulfate) is ground with coal powder in oil to form an iron-coal slurried catalyst. Subsequently, the catalyst is mixed with heavy oil to form feedstock for hydrogenation reaction.
The catalyst can be substantially dispersed into heavy oil. However, metal iron has only a little hydrogenation activity and coking is serious in the reaction process. Otherwise, the coal added as catalyst becomes coking support in the process, resulting in a lot of oil-insoluble solids in the product, thus, it brings much difficulty in separation and after-treatment, besides, the solid particles also wear the pipes and device.
U.S. Pat. No. 4,637,870 discloses a hydroconversion process, wherein phosphoric acid is added to an aqueous solution of phosphomolybdic acid. The phosphoric acid-phosphomolybdic acid aqueous solution is mixed with a hydrocarbonaceous material to form a catalyst precursor concentrate. The precursor concentrate is dehydrated, vulcanized, then mixed with heavy oil and residuum feedstock and introduced into a reactor to perform hydrogenation reaction. In the patent, it is mentioned that commercially available phosphomolybdic acid typically contains an atomic ratio of P/Mo ranging from about 0.08:1 to 0.01:1. If the phosphoric acid is added to the phosphomolybdic acid in an amount to provide an atomic ratio of P/Mo in the solution ranging from 0.12:1 to 0.45:1, coking can be obviously decreased (as shown in the examples, from 5.06% to 1.78%). In practice, however, this level of coking is still too high. Moreover, it is very inconvenient to premix the catalyst with hydrocarbonaceous material and predehydrate the catalyst before introducing it into a reactor in practical operation.
U.S. Pat. No. 4,637,871 discloses a hydrocoversion process utilizing an aqueous solution of phosphomolybdic acid as catalyst In this process, the aqueous solution of phosphomolybdic acid must comprise less than 5 wt % molybdenum. If the content is higher than 5 wt %, coking will remarkably increase. The speed and degree of hydrogenation reaction depend on the concentration of active metals in the reaction system. If an aqueous solution of phosphomolybdic acid having low concentration is used, a lot of water will be introduced into the catalyst-oil system in order to reach a proper concentration of active metals in the reaction system.
U.S. Pat. No. 5,039,392 discloses another modified process on the basis of the two patent process mentioned above, in which element sulfur is used as a vulcanizing agent to vulcanize the catalyst precursor concentrate, in order to simplify the preparation of the precursor concentrate. But, the following steps are still necessary: dispersing the aqueous solution of the catalyst into hydrocarbonaceous material, dehydrating, vulcanzing, adding it into feedstock and introducing into a reactor to perform the reaction. It is mentioned in the description that the amount of catalyst used is in the range of 50 to 300 ppm, however, an amount of 208 ppm is used in every example. In all examples, coke yields (solid product yield) are about 2.0 wt %, at least 1.8 wt %. Obviously, it is too high to be acceptable in practical operation.
High coke yield is a common problem in other similar techniques. therefore, it is necessary to propose a new technique to further lower the coke yield in catalytic hydrogenation of heavy oil and residuum.
In order to overcome these problems, the inventors have concentrated their research on the development of a catalyst which can further decrease the coke yield in catalytic hydrocracking of heavy oil and residuum, and has no disadvantagous effect on producing lower boiling products in the hydrocracking reaction.